Various drugs, biochemicals, antibiotics and the like are produced on a large scale in industrial fermentation plants. Fermentors are the basic tool in such production processes, and in most processes the content of the fermentor must be agitated and thoroughly mixed. Nowadays there is generally used a stirred baffled upright cylindrical fermentation vessel provided with central agitation means. The agitator is used for dispersing incoming air in the reaction or culture medium in the form of fine bubbles. Such bubbles must be moved from the area of the impeller to other regions of the fermentor. When used in systems comprising microorganisms, fungi, etc. the agitation attained mixing of same with the nutrient medium and causes local turbulence, causing adequate turbulence to prevent the dispersion from settling.
For this purpose different types of stirring means are used, mainly impellers with flat stirring paddles, or marine-blade impellers when reduced shear stress is required, usually with baffles around, for increasing turbulence and to prevent swirling of the culture medium.
The main drawback of the known systems is that when a dispersed phase is suspended, or in rise, or fall, the mass transfer coefficients are almost completely unaffected by mechanical power.
Little advantage can be gained by increasing the agitation intensity above the point at which the particles are freely suspended, while impellers should secure suspension of particles, thorough mixing, minimum shear and minimum power consumption.
It was found that the most economic use of power consumption for dispersion is obtained by small low-drag impellers, but this diminishes the rate of dispersion. Even with high speed impellers there occur stagnant regions above the impeller and below it and in the vicinity of the baffles. As the viscosity of the fluid increases, there will be a decrease of fluid velocities and the stagnant zone will increase in size.
Moreover, the tip speed of the impeller causes considerable cell damage and the shearing action occurs around the whole impeller. In many systems changes occur in rheological properties during the course of fermentation, influencing the degree of agitation and mixing, increasing power consumption.